How to reapply symmetry in sculpting?
If you are using Dyntopo, then just click the "Symmetrize" button under the Dyntopo settings panel. Just make sure that the "Direction" menu is set correctly. The default is "-X to +X". If the side that you want to keep is in the "-X" direction (away from the red arrow on the manipulator) then leave it how it is. If not, change it so that the first direction is the side you want to keep, and the second direction is the side that got messed up. This way, you don't have to leave your workflow and you can stay in Sculpt Mode.